This is my take on the unpredictability of SP movementsI think...

This is my take on the unpredictability of SP movements

I think the explanation is similar to pouring sand on mound and asking when will it collapse ? (except perhaps in reverse)
the grains are all sitting on each other and effect each other - sometimes the mound can rise quite high sometimes not
Traders are each watching what the others do at some point suddenly a snowball effect develops into a surge - had a few sell orders been done earlier maybe the surge would not have happenned

Others will probably have more deterministic reasons but I suggest that if you could pick these things even with 60% accuracy you'd be a billionaire